1. Field of the Invention
This invention relates to optical communication systems. More particularly, this invention relates to a method and apparatus for looping-back an optical signal used in optical communications.
2. The Prior Art
Continuity of optical fiber transmission lines is often tested using an Optical Time Domain Reflectometer (OTDR), which sends an optical signal into an optical fiber and then senses any reflections off of discontinuities, such as breaks, returning on the optical fiber to the OTDR. Use of an OTDR interrupts the normal transmission of data and does not allow for diagnostic analysis of other aspects of an optical network during the OTDR tests. Furthermore, OTDRs are expensive pieces of test equipment. Therefore, cost limitations prevent the simultaneous use of OTDRs on many different transmission lines of an optical network. Furthermore, OTDR detection of signal discontinuities may be difficult due to normal attenuation of the OTDR signal in the case of long transmission lines. OTDR's also have the disadvantage of not being able to perform several important functions, including: testing transmission line integrity; performing round trip performance analysis; injecting and detecting errors; and, testing higher communications layers.
Loop-back, in which a signal is sent down one optical fiber and returned on another optical fiber, is a less costly method of detecting discontinuities than using OTDR's. However, conventional loop-back systems require the entire transmission line be dedicated to the loop-back process during a discontinuity test or a performance test. Furthermore, loop-back tests are limited in range due to the fact that the test beam may be significantly attenuated due to the added optical path length introduced in the loop-back process.
Nowhere does the prior art disclose a method or apparatus for employing loop-back in a discontinuity test that allows continued use of the transmission line for the transmission of data and that uses normal data for the discontinuity test and that regenerates the test beam.